The present invention relates generally to cellular and other types of wireless communication systems, and more particularly, to code division multiple access (CDMA) systems which use multiple-tone modulation to achieve a wide-band signal with minimal intra-cell and intra-sector interference.
Of particular interest in cellular systems and other types of wireless communication systems are bandwidth-efficient multiple access schemes. Typical systems comprise a plurality of cells or designated regions, a base station associated with each cell and a plurality of mobile units. These systems require modulation schemes which efficiently use an allocated frequency band so that a maximum number of mobile units can be accommodated with a minimum amount of interference. Different modulation schemes have been developed for transmitting information signals from a mobile unit to a base station, (i.e., reverse or uplink direction) and from a base station to a mobile unit (i.e., forward or downlink direction). Communication in the uplink direction is particularly difficult because a base station must be able to receive, and distinguish among all of the information signals transmitted from those mobile units located within its particular cell. The quality of transmission of the information signals in the uplink direction depends in part on the amount of bandwidth allocated for transmission and the number of mobile units to be accommodated in a given cell.
Among the modulation schemes proposed is frequency hop spread spectrum modulation. Each mobile unit is assigned a so-called hopping sequence which modulates the information signals transmitted by the unit at a particular set of frequencies for a predetermined amount of time and then "hops" to another set of frequencies. (The number of frequencies in a set could be as small as "1").
An example of a frequency hopping system is discussed in U. Timor, "Multi-Tone Frequency-Hopped MFSK System for Mobile Radio", Bell System Technical Journal, Vol. 61No. 10, pp. 3007-3017, Dec. 1982. In the Timor system, each unit modulates information signals by a set of tones that are unique to the unit and to which a frequency-hopping sequence is applied. The signals received by the base station are a composite of the tone sequences assigned to each of the mobile units within a cell. Each link between a mobile unit and a base station is identified by an address word. The base station periodically performs a spectral analysis of the received signals. Based on the spectral analysis, the base station generates a frequency-time received energy matrix. A decoded matrix for a particular mobile unit is obtained by subtracting the address word from the matrix. Information signals are transmitted in the downlink direction from the base station to the mobile unit by modulating the information signals by the tone set assigned to the mobile unit. This system, however, is complicated by the requirement that the base station continually update the particular frequency-hopping sequence applied to the information signals transmitted by each mobile unit.
A more common modulation scheme is CDMA. CDMA systems broadcast all channels in the system in a common frequency band. Therefore, the time and frequency domains are shared by all the units within the cell simultaneously. One particular type of CDMA which is gaining widespread use is that of direct sequence CDMA (DS-CDMA). In DS-CDMA systems, information signals from each mobile unit are multiplied by a pseudo-noise (PN) sequence prior to RF modulation. Each mobile unit uses a unique PN sequence, referred to as a signature sequence. As a result, it is possible for the base station to isolate, and therefore recover the information signals from, the various mobile units.
In general, the extent to which each mobile unit is able to be assigned a o unique PN sequence is a function of the extent to which the PN sequences assigned to a cell are mutually orthogonal. Orthogonal signals are signals which have a cross-correlation coefficient of zero. In a given DS-CDMA cellular system with a given bandwidth and a large number of units, it may not be possible to provide for all units within a cell a set of signature sequences which are mutually orthogonal to each other. If a completely mutually orthogonal set of signature sequences is not available for all units in a cell, multiple access interference results. This may be thought of as a type of "cross-talk" which results from an inability to completely isolate a desired information signal from all other transmitted signals in the cell.
The multiple access interference seen by a particular unit is approximately proportional to the number of units in the DS-CDMA system. Because of the growing popularity of cellular and other wireless systems, there is a need to maximize the number of mobile units capable of transmitting in a cell without producing a corresponding increase in the degree of multiple access interference or other communication errors.